1. Field of the Invention
The present invention relates to a weld nut welding apparatus. More particularly, the invention pertains to a weld nut welding apparatus capable of welding a weld nut to a plate material as well as forming a bore in the plate material after the welding operation, the bore being aligned with an axis of a bore in the weld nut.
2. Description of the Related Art
Referring first to FIG. 1, there is shown a conventional welding apparatus which is used to weld a weld nut to a panel employed as a plate material. A panel 3 which has previously been formed with a bolt receiving bore 1 for a weld nut in a pressing process is placed on a lower electrode 5 and properly positioned thereon. Thereupon, a guide pin 9 which is being biased upwardly by means of the compressed air supplied from an air inlet 7 which is provided in the lower electrode 5 projects upwardly from the bore 1 of the panel 3. In other words, the guide pin 9 enables the panel 3 to be properly positioned on the lower electrode 5. Then, a weld nut 11 is placed on the panel 3 while being guided by the guide pin 9 which projects from the panel 3, and the weld nut 11 and the panel 3 are pressed between an upper electrode 13 and the lower electrode 5 in such a manner as to be clamped therebetween. Thereafter, current is supplied to the weld nut 11 and the panel 3 from a known welding power source (not shown), whereby the weld nut 11 is welded to the panel 3. It is to be noted that the reference numeral 11a in FIG. 1 denotes a projection.
Thus, this type of conventional weld nut welding apparatus requires a bolt receiving bore to be previously formed in the panel. For this reason, even in the case of panels which are pressed by the use of the same die (referred to as "panels with the same configuration", hereinafter), for example, a panel which is employed as one constituent element of an automotive body, when the number of weld nuts required or the welding positions thereof differ for each of various types of vehicles, the following problems are experienced.
(1) It is necessary to change the number and positions of bolt receiving bores for weld nuts for each of various types of vehicles even for panels with the same configuration. In consequence, the number of required parts increases, and the number of pallets for required parts increases correspondingly, which fact leads to a reduction in the flexibility of the assembly line arrangement.
(2) To prevent such increase in the number of required parts, it may be possible to adopt a method in which panels with the same configuration have previously been formed in a pressing process with that number of bolt receiving bores which equals the largest number of bores respectively required for various types of vehicles, unnecessary bores being closed with grommets, or a method in which weld nuts are first welded at all the bores and then unnecessary nut bores are filled with a caulking material. However, employment of such methods disadvantageously involves a wasteful use of time in mounting grommets and an increase in cost as the result of providing additional grommets, or an increase in weight as the result of welding unnecessary nuts together with the risk of the quality of the finished panel being deteriorated by the existence of unnecessary bores.